Unusual behavior of epicyclic frequencies around rapidly rotating compact stars

We report on numerical calculations of orbital and epicyclic frequencies in nearly circular orbits around rotating neutron stars and strange quark stars. The FPS equation of state was used to describe the neutron star structure while the MIT bag model was used to model the equation of state of strange quark stars. The uniformly rotating stellar configurations were computed in full general relativity. We find that the vertical epicyclic frequency is very sensitive to the oblateness of the rotating star. For models of rotating neutron stars of moderate mass, as well as for strange quark star models, the sense of the nodal precession of test particle orbits close to the star changes at a certain stellar rotation rate. These findings may have implications for models of kHz QPOs.